1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device capable of preventing disclination and improving the aperture ratio.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices have advantages such as low voltage operation, low power consumption, light weight, slim profile, and full color. The LCD device is widely used as a display for watches and calculators, computer monitors, television monitors, television sets, and hand-held terminals.
The TN (Twisted Nematic) mode LCD device has a problem in that the viewing angle is narrow. Accordingly, the technologies for increasing such a viewing angle are being studied. Technologies with wider viewing angles include In-Plane Switching (IPS) mode LCD devices and Vertical Alignment (VA) mode LCD devices.
The IPS mode LCD device widens the viewing angle by forming a pixel electrode and a common electrode together, on a thin film transistor (TFT) substrate. The liquid crystal is operated by a lateral electric field in the horizontal direction. However, the IPS mode LCD device has a problem in that the aperture ratio is lower due to its structural characteristics.
The VA mode LCD device uses a negative-type liquid crystal having a negative (−) dielectric anisotropy. Namely, when a voltage is not applied, a direction along a longer axis of a liquid crystal molecule is arranged perpendicularly with respect to the plane of an alignment layer. When a voltage is applied, the liquid crystal molecule is arranged in parallel with the plane of the alignment layer. An image is displayed by adjusting light transmittance using this property.
The VA mode LCD device distorts the electric field applied to the liquid crystal layer by having, on the substrate, an auxiliary electrode, an electric field guide window, and a dielectric rib. It aligns a director of the liquid crystal molecule using the distorted electric field to a predetermined direction, thereby widening the viewing angle.
FIG. 1 illustrates a plan view of a unit pixel of a VA mode LCD device according to the related art, and FIG. 2 illustrates a cross-sectional view taken along line I-I′ of FIG. 1. Referring to FIGS. 1 and 2, the VA mode LCD device includes: first and second substrates 1 and 2; a gate line 7 and a data line 9 respectively formed in horizontal and vertical directions on the first substrate 1 to define a pixel region; a pixel electrode 13 divided into a plurality of electrodes by an electric field guide window 14 formed on the pixel region; an auxiliary electrode 11 formed on the same layer as the gate line 7; a gate insulating layer 3 formed on the first substrate 1 including the gate line 7; a passivation layer 5 formed on the gate insulating layer 3 including the data line 9; a black matrix layer 4 formed on the second substrate 2; a color filter layer 6 formed on the second substrate 2 including the black matrix 4; a common electrode 8 formed on the color filter layer 6; a dielectric rib 10 formed on the common electrode 8; a liquid crystal layer 20 interposed between the first and the second substrates 1 and 2; a first alignment layer 15 formed on the passivation layer 5; and a second alignment layer 12 formed on the common electrode 8.
The first and the second alignment layers 15 and 12 may be alignment-processed using a polyamide or polyimide-based compound, polyvinylalcohol (PVA), polyamic acid or the like. A multi-domain pixel that is alignment-processed in a plurality of directions, may be formed on any of the first and the second alignment layers 15 and 12. The LCD device that is alignment-processed in the multi-domain in this manner is called a multi-domain vertical alignment (MVA) mode LCD device. The VA mode LCD device that will be described below refers to a MVA mode LCD device.
In the VA mode LCD device according to the related art, when an electric field is generated between the pixel electrode 13 formed on the first substrate 1 and the common electrode 8 formed on the second substrate 2, a fringe field is formed by the electric field guide window 14 and the dielectric rib 10 so that the liquid crystal molecules are aligned differently depending on the fringe field, whereby the viewing angle is compensated. At this time, the alignment angle or the direction of the liquid crystal can be controlled by using the voltage applied to the auxiliary electrode 11 formed on the first electrode 1.
However, in the VA mode LCD device, disclination is generated at the electric field guide window 14 or the dielectric rib 10. Accordingly, the desired image is not displayed on the part where the disclination is generated and image quality is thus deteriorated.
Also, because the distance between the electric field guide window 14 and the dielectric rib 10 is narrow (about 25 μm) in the VA mode LCD device of the related art, there has been a problem in that the aperture ratio deteriorates and a stable texture is hard to obtain.